Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. The source of the acoustic energy can be generated by explosions, air guns, vibrators, and the like. The energy source is positioned on or near the surface of the earth. Each time the energy source is activated it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be detected at many locations on the surface as a function of travel time. The sensors commonly used to detect the returning seismic energy include geophones, accelerometers, and hydrophones. The returning seismic energy is recorded as a continuous signal representing displacement, velocity, acceleration, or other recorded variation as a function of time. Multiple combinations of energy source and sensor can be subsequently combined to create a near continuous image of the subsurface that lies beneath the survey area. One or more sets of seismic signals may be assembled in the final seismic survey.
In order to develop a complete 3D or 4D seismic survey, acquisition and comparison of multiple seismic readings from a variety of sources are used to refine and enhance previous data increasing signal strength and accuracy, removing false data, background noise, and artifacts, ultimately increasing the signal to noise ratio and thus increasing the resolution of the data.
Previously, data were refined by overlaying two or more complete seismic datasets. Nickle (U.S. Pat. No. 6,640,190) describes collecting two time-lapsed sets of seismic data and generating a new data volume indicating the amounts and direction (upwards or downwards) by which the samples of the first seismic data set have to be translated in order to arrive at a representation that best resembles the second seismic data set. Calvert (U.S. Pat. No. 6,906,982) describes a method of eliminating multiple surface reflections by calculating the subsurface reflections for the original dataset. Naess (US2005219948) describes using a GPS system for 4D seismic surveys where only arrays that are most centered on the planned source point are fired where the center of the total source is positioned by GPS. Semb (US2007247971) describes repeated marine seismic surveys while adjusting the seismic source to align with the previous source. Brain, et al. (WO2006054181) uses the cross correlation between a base survey trace and new survey trace to determine if the trace(s) should be included in the 4D analysis. Robertsson, et al. (US2008015783) time lapse seismic surveys using interpolation from baseline seismic data to acquired seismic data. Aarre, et al. (GB2437390) identify displacement of a first and second seismic trace by aligning key features and using the change in position of the key features to calculate the change in position for the entire dataset.
Unfortunately, the previous methods all require the direct measurement of “substantially” the same dataset. When retrieving subsequent datasets, the source and receiver positions from one or more prior surveys are loaded into navigational packages that attempt to replicate source and receiver positions. Currently, a “feel good factor” is computed from how accurately and completely the current survey replicates the geometry of the prior survey. Due to current variations, equipment variations, ocean conditions, and the like, a high level of infill shooting is required to replicate all the prior source and receiver positions accurately. Consequently, costs and time are significantly increased for each “repeat” survey required to obtain a complete survey.
The “feel good factor” is commonly calculated in the industry from change in distance from shot point to shot point and from receiver to receiver for every trace in the composite bin. The correlation is normally considered good if the sum is less then 50 m for all traces in a composite bin. While efficient to calculate, this approach is not technically correct and requires a prohibitively expensive amount of data collection to work properly.
During a 4D seismic project, one of the most expensive aspects of seismic survey acquisition is acquiring the infill data to properly populate the survey bins with sufficient useable data to process each survey. The conventional industry standard approach is to attempt to duplicate the source and streamer positions of the original survey as closely as possible with each subsequent survey. In order to increase resolution in seismic data and reduce the time and amount of data required to infill 3D and 4D surveys, a new method of seismic data collection is needed.